1. Technical Field of the Invention
The invention relates in general to packet switched communications transport network. In particular, and not by way of limitation, the present invention directed to a method, a node and a network for a Multi-Protocol Label Switching (MPLS) network.
2. Description of Related Art
The exponential growth of the Internet over the past several years has placed a tremendous strain on the service provider networks. Not only has there been an increase in the number of users but there has been a multifold increase in connection speeds, backbone traffic and newer applications. Initially ordinary data applications required only store and forward capability in a best effort manner. The newer applications like voice, multimedia traffic and real-time e-commerce applications are pushing toward higher bandwidth and better guarantees, irrespective of the dynamic changes or interruptions in the network.
To honor the service level guarantees, the service providers not only have to provide large data pipes, but also look for architectures which can provide QoS guarantees and optimized performance with minimal increase in the cost of network resources.
With the advancement of the convergence of multi-service network onto a common Internet Protocol (IP) backbone, the resilience properties of IP-based transport networks receive increasing attention.
Investigating the resilience properties of multi-service IP/MPLS networks, an outcome of an important observation relating to the availability of the transport network was that the transport should be considered unavailable not only if it does not provide connectivity but also if it provides connectivity but the provided performance is so poor that sessions cannot be retained. One common cause of performance degradation is network congestion. Performance in terms of delay, jitter and packet loss is mostly impacted by congestion in an overloaded network segment.
In order to cope with the overload, networks are typically over-dimensioned so that under normal circumstances the probability of congestion is minimal. However, re-routing due to node or link failures is a very likely reason of congestion and so of an unavailable network.
After a failure, the protection and restoration mechanisms are responsible to restore connectivity as fast as possible so that ongoing sessions are not interrupted. While pure IP routing protocols are able to re-calculate the routes in the order of a few seconds, this fail-over time may not be enough for certain applications. If quicker fail-over is required, typically protection-switching methods are implemented where packets are quickly diverted to pre-established backup paths using MPLS backup Label Switched Paths (LSPs) in MPLS/IP networks.
MPLS technology enables Service Providers to offer additional services for their customers, scale their current offerings, and exercise more control over their growing networks by using its traffic engineering capabilities. While the MPLS solution is fast, it has a problem that if there is a failure in the backup path, and there is no more backup configured, connectivity may be lost. An IP routing protocol on the other hand could still find another alternative path after re-calculating the routing table. Therefore, instead of protection with static end-to-end backup path, MPLS networks often apply local protection switching, called Fast Re-Route (FRR) in combination with head-end re-routing of the MPLS tunnels. Summarised, the operation is such that each router has a so called detour LSP configured for each outgoing link and neighbour node. If the link fails, the router detecting this failure can immediately activate the detour, which can be used as long as the head-end router of the edge-to-edge LSP learns about the failure and re-calculates a new edge-to-edge path dynamically based on the IP routing protocol.
However, there are a lot of inefficiencies with respect to the related art.
Pure IP routing protocols e.g. Open Shortest Path First (OSPF) or Intermediate System to Intermediate System (IS-IS) do not require any additional configurations for traffic rerouting in case of failures, but have typically slow convergence properties after failures (in the order of 1 second or more), as it was mentioned before, and they can only calculate the shortest path, which can then easily become congested if more end-to-end routes will use the same link or network segment. So there is no Quality of Service (QoS) guarantee on the protection path, i.e., congestion may occur after failover. In cases of severe congestion this will even cause availability problems. Traffic engineering with off-line optimization of link weights to provide an optimized network load even after failures has proven to be a very difficult task which makes a scalability problems for large networks.
MPLS protection switching on the head-end of the tunnels pre-configured with bandwidth reservation gives a prompt solution to the congestion problem, but it is not resilient to multiple failures because if there is a failure also on the backup path, connectivity cannot be restored in the absence of further backups. Although the fail-over times are smaller than in the case of plain IP rerouting, they are still considerably larger than the fail-over times for the legacy networks, due to the fact that the failure notification has to reach the head-end node to get acquainted with the failure and re-direct the traffic on the protection path. Moreover, in large networks the big number of edge nodes leads to a very high number of edge-to-edge primary and backup LSPs to configure. The solution how to provide short fail-over times has been given by the MPLS-FRR architecture, since in this case the failure notification does not have to reach the head-end node, as the node observing the failure on the directly connected link, referred to as the Point of Local Repair (PLR) has the responsibility to re-direct the traffic on the detour paths, however the scalability properties of the concept are insufficient, since a multitude of detour paths are in principle needed to protect for all possible failures. A solution for scalability was offered, however, by making possible merging of detour paths with the original LSP and the possibility for so-called facility backups, which protect a given element with the same bypass tunnel for all different LSPs using that element. Management-wise the solution for scalability was offered by the possibility of automated setup of detour paths during the setup of the primary LSPs as it was described in IETF RFC 4090, “Fast Reroute Extensions to RSVP-TE for LSP Tunnels”, May 2005; and “Extensions to RSVP-TE Fast Reroute”, Internet Draft, December 2005. As it is seen, MPLS-FRR represents a trade-off in the sense that QoS can be guaranteed by bandwidth reservation also on the detour paths. The price for bandwidth reservation is however a substantially reduced utilization of network resources.
Other disadvantages of the prior art are disclosed by M. S. Kodialam and T. V. Lakshman, in “Dynamic Routing of Locally Restorable Bandwidth Guaranteed Tunnels Using Aggregated Link Usage Information,” in Proc. of Infocom, April 2001, pp. 376-385; and by S. Raza, F. Aslam, Z. A. Uzmi, “Online Routing of Bandwidth Guaranteed Paths with Local Restoration using Optimized Aggregate Usage Information”, Proceedings of IEEE ICC'05 Communications QoS, Reliability and Performance Modeling Symposia, 2005, proposing usage of on-line constraint-based optimization algorithms based on some lightweight link reservation information signalling. Although claimed to be very efficient, the methods solve the low utilization problem only partially, as they are all based on bandwidth reservation of backup paths, due to which an enormous amount of bandwidth is wasted.
It is an objective with this invention to improve the solutions described above by providing re-routing traffic flow in a packet switched communications transport network, which makes a better usage of the network resources.